The feeling one gets after eating a large meal from McDonalds, together with a large sugary soda and all the extra condiments. It's a very nervous feeling, where your stomach feels uneasy and you have the McTwitches. It's a term coined by Morgan Spurlock in his documentary Super Size Me.

As Morgan himself says in his hit documentary, while eating a double-quarter pounder super-sized meal:

"See, now's the time of the meal when you start getting the McStomach ache. You start getting the McTummy. You get the McGurgles in there. You get the McBrick, then you get the McStomach ache. Right now I've got some McGas that's rockin'. My arms... I feel like I've got some McSweats goin'. My arms got the McTwitches going in here from all the sugar that's going in my body right now. I'm feeling a little McCrazy."

The most delicious thing anyone could order from McDonald's. Here's the step by step ordering process:
Step 1: Order a McDouble from the dollar menu, add Mac Sauce.
Step 2: Order a 4pc chicken nugget, no sauce necessary.
Step 3: Open the McDouble and insert the nuggets in between the two beef patty's (biting of nuggets might be needed in order to fit more efficiently)
Step 4: Reassemble and enjoy the most delicious burger you'll every stick in your mouth hole

Yo dude, i just had a McCrazy, i'm never ordering a normal burger again!